Djinns exist, and it is true that some people use witchcraft to control djinns.
Allah (swt) has given mankind and djinns the freedom to choose between actions.
If anyone says he has control over djinns, he is doing what he isn't allowed, so he must also have used some non-sharia method to gain this control.
However, there are halal and sharia methods, sunnah, to get rid of djinns and shayateen.
See this:protection from and removal of djinns and shayateen